Advanced Configuration and Power Interface Specification

Thermal Management
11
Thermal Management
This section describes the ACPI thermal model and specifies the ACPI Namespace objects OSPM
uses for thermal management of the platform.
11.1 Thermal Control
ACPI defines interfaces that allow OSPM to be proactive in its system cooling policies. With OSPM
in control of the operating environment, cooling decisions can be made based on the system’s
application load, the user’s preference towards performance or energy conservation, and thermal
heuristics. Graceful shutdown of devices or the entire system at critical heat levels becomes possible
as well. The following sections describe the ACPI thermal model and the ACPI Namespace objects
available to OSPM to apply platform thermal management policy.
The ACPI thermal model is based around conceptual platform regions called thermal zones that
physically contain devices, thermal sensors, and cooling controls. Generally speaking, the entire
platform is one large thermal zone, but the platform can be partitioned into several ACPI thermal
zones if necessary to enable optimal thermal management.
ACPI Thermal zones are a logical collection of interfaces to temperature sensors, trip points, thermal
property information, and thermal controls. Thermal zone interfaces apply either thermal zone wide
or to specific devices, including processors, contained within the thermal zone. ACPI defines
namespace objects that provide the thermal zone-wide interfaces in Section 11.3, “Thermal
Objects”. A subset of these objects may also be defined under devices. OS implementations
compatible with the ACPI 3.0 thermal model, interface with these objects but also support OS native
device driver interfaces that perform similar functions at the device level. This allows the integration
of devices with embedded thermal sensors and controls, perhaps not accessible by AML, to
participate in the ACPI thermal model through their inclusion in the ACPI thermal zone. OSPM is
responsible for applying an appropriate thermal policy when a thermal zone contains both thermal
objects and native OS device driver interfaces for thermal control.
Some devices in a thermal zone may be comparatively large producers of thermal load in relation to
other devices in the thermal zone. Devices may also have varying degrees of thermal sensitivity. For
example, some devices may tolerate operation at a significantly higher temperature than other
devices. As such, the platform can provide OSPM with information about the platform’s device
topology and the resulting influence of one device’s thermal load generation on another device. This
information must be comprehended by OSPM for it to achieve optimal thermal management through
the application of cooling controls.
ACPI expects all temperatures to be represented in tenths of degrees. This resolution is deemed
sufficient to enable OSPM to perform robust platform thermal management.
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